Dimensionless physical constant
In physics, a dimensionless physical constant, sometimes called a fundamental physical constant, is a physical constant that is dimensionless. It has no units attached and has a numerical value that is independent of the system of units used. Perhaps the best-known example is the fine-structure constant, α, which has approximate value of 1⁄137.036.
The term fundamental physical constant is also used to refer to universal but dimensioned physical constants such as the speed of light c, vacuum permittivity ε0, Planck constant h, and the gravitational constant G. Increasingly, physicists reserve the use of the term fundamental physical constant for dimensionless physical constants that cannot be derived from any other source.
There is no exhaustive list of such constants. But it is meaningful to ask about the minimal number of fundamental constants necessary to determine a given physical theory. Thus, the Standard Model requires 25 physical constants, about half of them the masses of fundamental particles (which become "dimensionless" when expressed relative to the Planck mass or, alternatively, relative to the electron mass along with the gravitational coupling constant).
Fundamental physical constants cannot be derived but have to be measured. Development in physics may lead to either a reduction or an extension of their number: discovery of new particles, or new relationships between physical phenomena, would introduce new constants, while on the other hand, the development of a more fundamental theory might allow the derivation of several constants from a more fundamental constant.
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